Electron multiplier



Jan. 13, 1959 R, H. ANDERSON ELECTRON MULTIPLIER 2 Sheets-Sheet 1 Filed001:. 24, 1955 HT ORA/E) INVENTOR. H A/VJEASO/V Roy/er BY I R. H-ANDERSON ELECTRON MULTIPLIER Jan. 13, 1959 Filed Oct. 24, 1955 2Sheets-Sheet 2 INVENTOR. fiwz/vr/i/llvomso/v United States. PatentELECTRON MULTIPLIER Robert H. Anderson, Princeton, N. J., assignor toRadio Corporation of America, a corporation of Delaware The presentinvention relates to improvements in electron multiplier tubes.

An electron multiplier is a device utilizing secondary electron emissionto amplify or multiply the electron current from a primary electronsource, such as a photocathode or a thermionic cathode. The usualelectron multiplier comprises a series or chain of secondary emittingelements, called dynodes, interposed between a primary electron sourceand an output collector or anode. The electrodes are constructed andarranged to form an electron optical system for directing primaryelectrons from the primary source onto the first dynode, releasingtherefrom several secondary electrons for each primary electron. Thesesecondaries are directed by the electron optical system onto the nextdynode where each produces more secondaries. This process is repeated ateach succeeding dynode or stage of the multiplier, thus producing agreatly multiplied electronic current from the final dynode to thecollector. The number of dynodes or stages may be from one to twenty ormore depending on the amount of amplification needed. Each succeedingdynode in the chain is maintained at a potential substantially higher,e. g. 100 volts, than the preceding dynode, to accelerate thesecondaries from element to element, and the dynodes are preferablyshaped to direct and focus the electrons emitted thereby to the nextdynode.

Electron multipliers are particularly useful for amplifying electroncurrents produced by weak signals, such as light, nuclear radiations orradio waves. When used for detecting and/or counting rapidly recurrentsignals such as nuclear particles, it is necessary that the multiplierhave sufiicient speed and a resolving time low enough to distinguishbetween successive signals or particles.

In addition, it is desirable that an electron multiplier experiencespace charge saturation at a high current level so that a high maximumcurrent signal may be obtained.

Accordingly, the objects of this invention concern the provision of animproved electron multiplier which operates with reduced transittime andreduced transit time spread and experiences space charge saturation at ahigh current level.

According to the present invention, an electron multiplier deviceincludes a primary electron-emitting surface such as a photocathode, atransition dynode for receiving electrons emitted by the primaryelectron-emitting surface and an electron multiplying chain of dynodesfor amplifying electrons emitted by the transition dynode. Themultiplier chain is preferably of the zigzag type having two rows ofdynodes with the dynodes staggered with respect to each other. Eachstage of the multiplier chain includes a dynode and an auxiliaryelectrode which is positioned adjacent to and in advance of the dynodeand which cooperates with the next auxiliary electrode in the oppositerow to form, in effect, a cylindrical lens which serves to focus andaccelerate 2' electrons onto successive dynodes and to accelerateelectron transit through the multiplier chain.

In the drawing:

Fig. 1 is'a sectional elevational view of a device including an electronmultiplier embodying the principles of the invention; and, i

Fig. 2 is an elevational view of the electron multiplier portion of thedevice of Fig. I removed from the envelope of Fig. 1 and enlargedin sizeand having a modified system of electrical connections.

Referring to the drawing and to Fig. 1, one type of device which mayembody the principles of the invention is a photomultiplier tube 10.Such a tube includes a tubular envelope 12 having a base 13 at one endand a comparatively large-area transparent face plate 14 at the otherend, on the inner surface of which plate is formed a photocathode 16adapted to emit photoelectrons when exposed to external radiationrepresented by the arrows 18.

The photocathode 16 may be of the alkali-antimony type or of any othersuitable type and may be formed by evaporation of the selected materialsonto the face plate. A conductive film 20 of aluminum or silver or thelike, extends along the inner wall of the envelope from the photocathode16 toward the opposite end of the envelope and serves to provideelectrical contact to the photocathode.

An electron optical system is provided within the envelope 12 adjacentto the photocathode 16 and is adapted to focus and accelerate thephotoelectrons to an electron multiplier 22 which serves to multiply thephotoelectrons. The electron optical system may be of any convenienttype and may include, for example, a tubular focusing electrode 24, apair of deflection plates 26, and a disk accelerating electrode 28having an aperture 30 through which electrons pass to the electronmultiplier 22.

According to the invention, the electron multiplier 22 includes atransition dynode 32 which is generally smoothly curved in the form of asection of a cylinder and has its concave surface facing the aperture 30in the disk electrode 28. The transition dynode is tilted at an angle tothe plane of the aperture so that it is inclined to face also toward thefirst dynode of a zig-zag multiplier dynode chain 33. The area of thetransition dynode 32 is preferably intermediate between the area of thephotocathode 16 and the first dynode of the multiplier chain.

' The multiplier chain 33 includes a plurality of electronemittingdynodes, 34, 36, 38, 40, 42, 44, 46, 48, and 50, and a collectorelectrode 52. The dynodes are arranged in two substantially straightrows, on opposite sides of any axial plane (not shown) extendingperpendicularly to the plane of Fig. l. The dynodes are staggered inposition with one dynode in one row being in advance of the next dynodein the other row so that, in effect, a zig-zag line 54 may be drawnbetween each dynode in one row and the next closest dynode in the otherrow. In elfect, this Zig-Zag line representsthe path followed byelectrons as they travel between successive dynodes in the zig-zagmultiplier chain. All of the dynodes of the chain 33, except the last,have the same area and shape, each being a section of a cylinder tiltedto face the next closest dynode in the opposite row, to promote thefocusing of electrons from one to the other. The last dynode 50 ispreferably generally cylindrical in form and partially surrounds thecollector elec trode 54 which is described in greater detail below.

According to the invention, auxiliary means are provided for focusingand accelerating electrons through the multiplier. The auxiliary meanscomprises curved non-emissive electrodes 56, 58, 60, 62, 64, 66, 68, 70and 71 disposed ahead of the dynodes 34, 36, 38, 40, 42,

44, 46, 48, and 50, respectively. Each of the auxiliary electrodesfollows the general contour of the dynode which it precedes and it isgenerally of shorter length than the dynode. Thus disposed, each pair ofadjacent auxiliary electrodes, for example, electrodes 56 and 58, 58 and60 and so forth, comprises a cylindrical electron lens.

The collector electrode 52 comprises a thin flat plate disposedsubstantially parallel to and just outside of the main path of electronflow from the dynode 48 to the last dynode 50. Thus,the collector doesnot intercept electrons traveling in this path. At the same time, thecollector presents a favorably large surface to the last dynode which isso curved that the secondary electrons emitted therefrom are focused onthe collector. Since the collector is a solid flat plate, electronswhich arrive there are not subject to oscillation as they are in agridtype collector.

Suitable arrangements for supporting the various electrodes of'theelectron-optical lens system and the multiplier chain are well known inthe art and any of these may be employed. As shown in Fig. l, themultiplier chain may be supported by a pair of fiat, parallel micaplates, one of which, 72, is shown, which may be secured to oppositeends of the electrodes and dynodes of the multiplier chain 22. Thelength of the transition dynode 32, each of the dynodes of themultiplier chain 33 and the collector 52 in the direction perpendicularto the plate of Fig. 1 may be several times the lengths of the wallportions shown in section in Fig. l.

The tube is provided with separate electric-a1 leads for each of thedynodes, auxiliary electrodes and collector electrode for applyingseparate D. C. potentials thereto. Perferably, these leads extendthrough the base 13 of the envelope 21. Only four leads 74 are shown forsimplicity, the rest being shown schematically as arrowtipped lines. Astypical examples of electrode operatingpotentials, the cathode 16 may beoperated at zero potential, the electrodes 24, 26 and 28 of the electronoptical system at about 100 volts, the transition dynode 32 may bemaintained at 100 volts and each succeeding dynode may be 100 voltshigher than the preceding dynode, in which case, with the number ofdynodes shown, the last dynode carries a potential of 1000 volts. Thecollector may be operated at a potential about 1000 volts higher thanthat of the last dynode, or about 2000 volts, in the example shown.

Fig. 2 shows an alternative arrangement for making electricalconnections to the multiplier portion 22 of the tube 10. The multiplieris shown enlarged and removed from the envelope. In this alternativearrangement, all of the auxiliary electrodes are connected within thetube envelope to points having the desired positive potential. Thus, alead 76 may be connected between the auxiliary electrode 56 and thedynode 38, a lead 78 between the auxiliary electrode 58 and the dynode40, a lead 80 between the auxiliary electrode 60 and the dynode 42, alead 82 between the auxiliary electrode 62 and the dynode 44, a lead 84between the auxiliary electrode 64 and the dynode 46, a lead 86 betweenthe auxiliary electrode 66 and the dynode 48, a lead 88' between theauxiliary electrode 68 and the dynode 50, and a lead 90 between theauxiliary electrode 70 and the collector 52. The auxiliary electrode 71,in this arrangement, is provided with a separate external lead 92. Inthis alternative arrangement, external leads, shown as arrowtipped linesare required only for the transition dynode, the dynodes of themultiplier chain, the collector electrode and the last auxiliaryelectrode.

What is claimed is:

1. An electron multiplier comprising a plurality of electron-emittingdynodes and a non-emissive auxiliary electrode adjacent to and inadvance of each dynode and constituting, in effect, a continuation ofthe surface thereof, and means connected to each auxiliary electrode forapplying a separate potential thereto.

2. An electron multiplier comprising a plurality of curved dynodes and acurved non-emissive auxiliary electrode in advance of each dynode andconstituting, in effect, a continuation of the surface thereof.

3. An electron multiplier chain including a plurality ofdynode-electrode pairs, each dynode-electrode pair comprising a curveddynode and a curved electrode adjacent to and in advance of it, eachelectrode of each pair constituting, in effect, a smooth continuation ofthe surface of each dynode of each pair.

4. An electronrnultiplier comprising a plurality of curved dynodesarranged in zig-zag relationship on opposite sides of an axial plane,and a curved electrode adjacent to and in advance of each dynode andcomprising, in effect, a continuation of each dynode.

5. An electron device comprising a source of elec trons, a collector ofelectrons, a plurality of electronemitting dynodes placed between saidsource and said collector, a plurality of two-part electron lenses eachbetween two difterent successive dynodes, one part of each two part lensbeing common to a succeeding two part lens, and connection means to eachlens part for applying a separate operating potential thereto.

6. An electron multiplier chain comprising a plurality of dynodesarranged in zig-zag relation in two rows oriented on opposite sides ofan axial plane, generally cylindrical electron lenses between successivedynodes in said chain, and separate electrical connections to the lensesfor applying separate operating potentials thereto.

7. The device defined in claim 6 wherein said primary source ofelectrons is a photocathode.

8. The device defined in claim 6 and including a collector electrodepartially surrounded by one of said dynodes and comprising a flat platedisposed parallel to and out of the path of electrons between said oneof said dynodes and another dynode.

References Cited in the file of this patent UNITED STATES PATENT S2,159,519 Brauer et al. May 23, 1939 2,200,722 Pierce et al. May 14,1940 2,702,865 Herzog -4 Feb. 22, 1955

